Some handheld radios such as a Secure Personal Radio (or Soldier Personal Radio) manufactured by Harris Corporation under the designation RF-7800S use a low cost in-phase/quadrature demodulator or similar demodulator device that has processing difficulties resulting from I/Q DC offsets and I/Q gain and phase imbalances in the received signal. FIG. 1 is a graph showing the output output spectrum of such a device when the input is a constant wave (CW) tone. The output spectrum includes an input, DC offset and conjugate imbalance image signal. As illustrated, there are a number of other signals besides the input signal shown at −100 kHz. The graph shows a DC offset at 0 Hz in the middle and a conjugate imbalance image at +100 kHz on the other side from the −100 kHz input signal. The question mark indicates a spur signal.
These other signals as part of the output signal spectrum, including those created by the I/Q DC offsets, I/Q gain imbalance and I/Q phase imbalance, degrade overall performance of a narrowband interferer (NBI) excision filter and associated filtering process typically incorporated as signal processing before demodulation. A preferred received signal, on the other hand, would have a flat spectrum except for at the input signal located −100 kHz on FIG. 1, for example.
The incorporation of the NBI excision filter is important because more commercial, national, and international users of these types of radios are migrating to the use of wideband networking waveforms such as the Soldier Radio Waveform (SRW), the Wideband Networking Waveform (WNW) and the Advanced Networking Wideband Waveform (ANW2) as non-limiting examples. These waveforms operate, for example, in bandwidths typically of about 5 MHz. These types of modern communication systems and radios as described operate in wider bandwidths in order to support the higher data rates desired by these more advanced users. Thus, the use of the NBI excision filter is important for processing signals prior to the demodulating of these waveforms.
An NBI excision filter can process a received communications signal such as shown in FIG. 1 that includes these described multiple, narrowband signals. Its processing and its performance, however, is compromised. The narrowband interferer excision filter works better when a fewer number of narrowband signals are processed. As shown in FIG. 1, for example, there are three signals when preferably there should only be one signal to process within the NBI excision filter. As a result, the performance in this narrowband interferer excision filter is degraded.
Some systems minimize the signal degradation that occurs in a narrowband interferer excision filter by using very high quality parts and associated high power parts, which are incorporated into better radio frequency (RF) designs. Although this would work in some cases, the device cost of design and manufacture is increased. Other designs work on the transmitted signal. For example, it is possible at the transmitter to use Cavers algorithms as a transmitter based compensation to improve the quality of a transmit signal. This type of algorithm works by exploring grid nodes using the Dijkstra algorithm in which each node is evaluated using a path search algorithm. The drawback, of course, is that this occurs only at the transmit side and not the receive side. Some proposed techniques process received samples to reduce related images and improve the narrowband interferences excision filter performance. For example, Roke Manor Research Ltd. has a receiver design that eliminates only DC offset. This type of design would not compensate for gain and phase imbalance.
It would be better to develop an integrated receiver design and process that receives signal samples before the narrowband interferer excision filtering occurs and reduce the components that are generated by a low cost I/Q demodulator chip. Further performance could be improved because there are fewer narrowband signals for the NBI filter to process. It is desirable to eliminate the related narrowband signals so that in effect the NBI excision filter sees only one signal to filter instead of the multiple signals as shown in FIG. 1, showing the DC offset and conjugate image that comes from the gain and phase imbalance.